Variable displacement axial piston pumps and motors generally include a rotatable cylinder barrel containing several pistons which reciprocate in mating piston bores, more or less, parallel to the axis of a drive shaft. One end of each piston is held against a tiltable swashplate. When the swashplate is tilted relative to the drive shaft axis, the pistons reciprocate within their bores and a pumping action occurs. Each piston bore is subjected to two main pressure levels during each revolution of the cylinder barrel. One pressure is a result of the load and is located on one side of the ramp of the tilted swashplate. The other pressure is normally much lower and is located on the other side of the swashplate ramp. As the piston bores sweep past the top and bottom dead center positions, a swivel torque is generated on the swashplate as a result of the reciprocating pistons and pressure carryover within the piston bores.
The swashplate is typically controlled using one or more actuators and a bias spring to offset the swivel torque. The swivel torque is quite high in today's high pressure axial piston units such that the actuators are quite large and may account for approximately 20% of the overall size of the pump or motor. Swashplate response and control response are limited because of the volumes of fluid that need to flow into and out of the hydraulic actuators and the total added inertia of the actuators. Moreover, such actuator system within the pump contributes from about 7-12% of the overall cost of the pump. These costs result from the number of pieces used in the actuators and the precision machining of several large pieces and the expense associated with assembly of the pump.
There have been at least two proposals to control the angle of the swashplate by using the pistons within the cylinder barrel instead of a separate actuation system. One such unit is disclosed in Japanese Utility Model Application No. 61-37882. Another unit is disclosed in U.S. Pat. No. 4,918,918. Both of those disclosures have control ports at the top and bottom dead center positions for communicating with the piston bores as they sweep past the dead center positions. The control ports are selectably communicated to the intake and discharge ports to control the pressure therein to modify the swivel torque imposed on the swashplate to control swashplate positioning.